1. Field of the Invention
The invention is related to a method for suspending particles, especially magnetically attractable particles and beads such as ferro-and/or paramagnetic particles, for example in a liquid mixture used for diagnostic or analytical purposes.
2. Description of Related Art
In the field of sample preparation and sample processing for analytical or diagnostic studies, processes are increasingly used depending on utilisation of magnetically attractable particles, to which either particularly biological target molecules or contaminants can bind. Magnetically attractable particles can be separated from the mixture they are suspended in by appropriate magnetic fields. This particularly applies to automated processes, thus allowing a great number of samples to be analysed in a short time without extensive steps of centrifugation. This allows a large sample turnover and permits to reduce considerably the complexity of extensive and particularly parallel studies. Important fields of application are the purification of biological or medical samples, generally the separation and isolation of particularly biological target molecules, medical diagnostics, and pharmaceutical screening methods for the identification of potential pharmaceutical agents.
Methods for the separation of magnetically attractable particles are disclosed for example in DE 44 21 058, DE 103 31 254, DE 10 2005 004 664, WO 94/18565, WO 99/42832, WO 02/40173, WO 2005/044460, U.S. Pat. No. 5,942,124 and U.S. Pat. No. 6,448,092. The basic principle of the methods described there depends on the fact that a separation apparatus, for example a magnetic bar, is immersed in a usually liquid mixture and that the magnetically attractable particles in the mixture are concentrated on the surface of the separation apparatus by effect of the magnetic field. Thereafter, the separation apparatus with the adherent particles is removed from the liquid.
The application of external magnetic fields for mixing and separating magnetic particles is described in WO 2006/010584. For this purpose, pole shoes are arranged around an especially designed mixing vessel, so that changeable magnetic fields can be produced.
For mixing particles it is also known to use mixing bars, setting the mixture in motion by rotation, as e.g. described in US 2006/0118494, and thereby whirling the particles in the mixture. However, rotational solutions are very extensive, particularly in automated parallel processing.
Particularly when the magnetic particles come into contact with multiple solutions during separation and/or purification processes, for example in binding or washing processes, there are often losses in yield of the target molecules binding to the magnetic particles or insufficient purification results, if the particles in the solutions or mixtures are not sufficiently suspended, but precipitate at the bottom. In addition, particles used in such processes per se show a high tendency for sedimentation. Therefore, efforts are being made in the described processes in order to keep the magnetic particles at least temporarily in the balance by mechanical mixing movements or to re-suspend the precipitated particles, respectively.
One problem appearing during practical application of the processes of the state of the art is that the particles coated with the target molecules, particularly the biological target molecules, or the contaminants no longer stick to the magnet as particles during production of a magnetic field and direct or indirect collection of the magnetic particles at the magnet, but rather as clumps or flakes, respectively. This results in that the particles can only be suspended badly and re-precipitate very quickly after being released from the magnet for example for washing the particles or eluting the adherent components. This can also lead to bad purification results.